Hmmm, I don't think it's lack of realism that bothers me with the Caesar's Legion scenario, more the fact that the great Caesar, scourge of the NCR, is such a cretinous bonehead come the endgame. He's so absurdly trusting. "I'll just assume that all that racket I heard was you following orders, no need to have anyone check". The man's an idiot, which makes it very difficult to want to side with him no matter what sort of character I'm rping.
I still think the Legion having an irrational case of "we don't wanna go down there" is at least a passable excuse for the PC to be drafted instead.
The man's a brutal tyrant who thinks nothing of literally decimating his own troops when they displease him, I don't think he'd be in the least bit squeamish about sending some grunts to near-certain doom, even if he'd rather not go down in the dark cellar himself. Still, maybe you're right, and I should just roll with the punches of another endgame scenario. It just seems so horribly contrived though.